I am facing the same problem. It only happens when I try to send emails to *@streamyx.com and *@tm.net.my (both under the same telco).

Usually when I sent an email to the above domains, I got the error email report within the next 1 minute.

I followed the instruction above and no longer getting the error email reports at least for the next 20 minutes. But the recipients did not receive the emails either. When I checked under WHM mail queue, all emails are there.